1. Field of the Invention
This invention relates to a filter of the type having a casing composed of two easing members.
2. Description of the Prior Art
FIGS. 11 to 14 illustrate a conventional filter apparatus of the above-described type. The filter apparatus is shown upside down for convenience' sake.
A casing 1 of the shown filter apparatus comprises lower and upper casing members 2 and 3 each of which has a concavity, as shown in FIGS. 11 and 12. The lower casing member 2 has a suction port 4 and the upper casing member 3 has an exhaust port 5. A filter 6 is formed by folding a sheet of filter medium back and has an opening 7 formed so as to correspond to the suction port 4 of the lower casing member 2, as shown in FIG. 13.
The lower and upper casing members 2, 3 are placed so that the concavities thereof face each other with the filter 6 being enclosed. A peripheral edge of the lower casing member 2 is put into a peripheral curled portion of the upper casing member 3, thereby being held in position so that a seal is provided. In this state, the peripheral edge of the filter 6 is held between the peripheral edges of the lower and upper easing members 2, 3 except at its folded portion. Since portions of the peripheral edges of the casing members 2, 3 holding the peripheral edge of the filter 6 are continuous with those not holding it as shown in FIG. 14, the upper casing member 3 is previously formed with a stepped portion 8 so that the folded portion of the filter 6 can be positioned.
The folded portion of the filter 6 thus needs to be positioned with respect to the stepped portion 8 in the above-described filter apparatus. This positioning complicates the steps of assembling the filter apparatus. Furthermore, when the casing 1 is formed of a metal, the sealing performance is reduced at the portions of the peripheral edges of the casing members 2, 3 not holding the peripheral edge of the filter 6 therebetween. Additionally, since the filter medium is folded back through holes formed in the filter 6 to place the interiors of the casing members 2, 3 in communication cannot be located at an optimum position.